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As part of a new study, shore crabs that were given a mild electrical shock responded in a way indicating they felt pain. Credit: Queen’s University Belfast

Can crabs feel pain? New research on the clawed crustaceans suggests the answer is yes.

A group of UK researchers came to this conclusion by examining the reactions of common shore crabs to mild electric shocks in a study released today in the Journal of Experimental Biology. The key to their finding is the distinction between the nervous system activity known as nociception and pain, which is defined as an unpleasant sensory and emotional experience. For years, many researchers assumed crustaceans such as crabs experienced the former, but not the latter.

Nociception—which differs from pain in that it isn’t subjective—is produced by the peripheral and central nervous systems in reaction to potentially tissue-damaging stimuli. All animals experience this reflex, including humans—for example, the nerve endings (called nociceptors) under our skin transmit a signal along our spinal cord to the brain when we touch a too-hot plate, and we automatically jerk our hands back.

For crabs, nociception provides immediate protection following a small electric shock, but it shouldn’t trigger any changes in its later behavior. That’s a job for pain—it helps organisms learn to avoid the harmful source in the future.

In this study, the crabs appeared to do just that. Ninety crabs were placed in a tank with two areas without a light source, one crab at a time. After the crabs scuttled toward the dark area they liked best, they were removed from the tank and exposed to a mild electric shock.

Following a rest period, each of the crabs was returned to the tank. Most of the crustaceans returned to the shelter they’d picked the first time. Those who had received a shock in the first round were zapped again, and when they were introduced into the tank for the third time, the majority moved to the other, presumably shock-free safe area. Crabs who hadn’t been shocked returned once again to their first-choice area.

Shore crabs chose on which side of the tank to seek shelter. Credit: Queen’s University Belfast

Dark hideaways, like under rocks along waterbeds, are important to these creatures because they offer protection from predators. After receiving the electric shocks, the decapods chose to trade in safety to avoid the unpleasant experience in the future.

“Having experienced two rounds of shocks, the crabs learned to avoid the shelter where they received the shock,” said study co-author Bob Elwood, an animal behavior professor at the School of Biological Sciences at Queen’s University Belfast, in a statement. “They were willing to give up their hideaway in order to avoid the source of their probable pain.”

So did the crabs remember the pain? The researchers say it’s possible, and previous work by Elwood and others supports the idea.

In a 2009 study with hermit crabs, wires attached to the creatures’ shells delivered small shocks to their abdomens, which they typically protect by crawling into empty mollusk shells. The only crabs to abandon their shells in search of others had previously incurred electric shocks, which researchers say means the crabs found the experience unpleasant—and perhaps ouch-worthy.

A new shell was then offered, and those crabs that had been shocked but remained in their original homes moved quickly toward the new option, investigated it for a shorter time and were more likely to make the switch than those who hadn’t been shocked. Experiencing shocks changed the hermit crabs’ motivation, much like the way we choose not to touch that hot plate again.

Such behavioral changes were also the subject by a 2007 paper by Elwood, with a different crustacean, the prawn. Various noxious stimuli introduced to prawns’ antennae elicited a reflexive tail flick. But after that, the prawns groomed their antennae and rubbed them against the side of their tanks, prolonged activities that, researchers say, signal the experience of pain.

While it’s impossible to explicitly demonstrate that crustaceans like crabs, prawns and lobsters feel pain, researchers hope these findings spur investigation of how the marine animals are handled in aquaculture and in the kitchen, where chefs often declaw or boil crabs alive.

Gastroptychus spinifer is capable of seeing UV light, researchers discovered. Image via NOAA Bioluminescence Team

A few years ago, when Tamara Frank, Sönke Johnsen and Thomas Cronin, a team of marine biologists, descended nearly half a mile to the ocean floor near the Bahamas in a tiny submersible, they were fairly stunned by what they saw: close to nothing. “We were surprised by how little bioluminescence is down there,” Frank told LiveScience. In one of the world’s first explorations of bioluminescence on the deep ocean floor, they found that, unlike in the open ocean, where scientists estimate that 90 percent of organisms produce bioluminescent light, just 10 to 20 percent of the creatures at the bottom of the ocean (mainly plankton) were capable of glowing.

When the team parked the submersible, shut the lights off and simply observed, though, they were amazed. “If you sit there with the lights out, you’ll see this little light show as plankton run into different habitats,” Johnsen said. “There is no substitute for actually being in that habitat to understand what it’s like to be those animals.” Over time, they identified several organisms that no one expected to glow that were generating light, including coral, starfish, sea cucumbers and the first-ever bioluminescent sea anemone, as described in a study published yesterday in The Journal of Experimental Biology.

Ophiochiton ternispinus, a species closely related to starfish, was found glowing on the ocean floor. Image via NOAA Bioluminescence Team

They also discovered that the several species of crabs inhabiting the ocean floor had a very unusual characteristic: As described in a concurrent paper published in the same journal, they found the first crabs ever identified as capable of seeing ultraviolet (UV) light.

While measuring the wavelengths of light produced by each of the organisms, the team noticed in particular the crabs’ skill at grasping plankton and other food to eat. “They just hang out in these plantlike things, and every so often—they have these amazingly long claws—they reach over and they’re clearly picking something off and bringing it to their mouths,” Frank said.

Intrigued, they tested the crabs’ vision for themselves. Using special equipment on the submersible, they suctioned the creatures into light-tight containers and brought them to the surface, then conducted an experiment aboard their ship. Flashing various colors and intensities of light at the crabs while using electrodes to monitor their eye movement, Frank discovered that all seven species tested were capable of seeing blue light. This wasn’t particularly surprising, as blue is the only color of light that can naturally penetrate down to the ocean floor as all other colors are filtered out by the water.

The second part of the experiment, though, was rather surprising. Two of the crab species they found, Eumunida picta and Gastroptychus spinifer, also moved their eyes in a way that indicated they could see green and ultraviolet light.

This raised an immediate question. “There is absolutely no UV and violet light coming down at that depth; it’s long gone,” said Johnsen. In that case, why on earth would the crabs have evolved to be capable of seeing it? Scientists have long assumed that organisms living on the nearly pitch black sea floor were colorblind, since there is so little color to be seen.

Their answer, for now, is only a hypothesis—but an extremely compelling one. “Call it color-coding your food,” said Johnsen. If the creatures can see green, blue and ultraviolet light, they might be capable of distinguishing between UV-emitting anemones and green-glowing toxic corals (which are not safe to eat) and blue-glowing plankton (which are the crabs’ primary food source).

“It is only a hypothesis. We could be wrong,” Johnsen said. “But we can’t think of another reason why an animal would use this ability to see UV and violet light because there isn’t solar light left.”

The crabs may use their color vision to avoid toxic anemones like Actinoscyphia sp, the venus fly trap anemone, which secretes a bioluminescent mucus (in blue) for defense. Image via NOAA Bioluminescence Team

Part of the reason we know so little about the seafloor environment, he says, is because of the difficulty in getting the funding for and access to a submersible necessary to conduct these sorts of observations. The researchers, though, say that learning about this habitat is a crucial first step in building support to protect it.

“The sea floor is three quarters of the earth’s area and the water column is over 99 percent of the earth’s liveable space, yet we know less about it than the surface of the moon,” Johnsen told the BBC. “I think people will only protect what they love, and they’ll only love what they know. So part of our job is to show people what’s down there.”

Horseshoe crabs spawning in the bay, by Nick Tucey (Fredericksburg, Virginia)

For a few weeks between late May and early June, horseshoe crabs in Delaware Bay storm the shoreline to spawn, and it’s a spectacle that spoke to the creative side of photographer Nick Tucey. “I wanted to capture the action of waves crashing upon a pod of horseshoe crabs,” Tucey says. “This photo is important to me because it captures an amazing wildlife event that occurs in the mid-Atlantic for only a brief time each year. My wife and I enjoyed watching and photographing this ancient species—which predate the dinosaurs—as they came ashore. We also assisted a number of horseshoe crabs that were stranded in the rocks and stuck upside down so they could crawl back into the Bay to continue their life-cycle.”

Tucey’s snapshot is one of 50 images selected as finalists in Smithsonian magazine’s 8th annual photo contest. You have until March 31, 2011 to vote for your favorite photographs, and the winner—along with the category and Grand Prize winners chosen by Smithsonian editors—will be announced July 1, 2011. And if you’re an aspiring shutterbug yourself, consider entering your work into the 9th annual photo contest, which is open for submissions until December 1, 2011.

Once a fiddler crab (above, a male Uca pugilator) gets big enough, he no longer needs to hide in a periwinkle shell (photo courtesy of flickr user cotinis)

Life can be tough for a fiddler crab. So many other creatures find them tasty: migratory birds, shrimp, fish, raccoons, turtles, even other species of crab. Adults, at least, can dig themselves a burrow and fight off predators. But juveniles don’t—or can’t—seek shelter in the sand. They can hide beneath vegetation, but that’s not always an option. What’s a little crab to do?

They use empty shells from the marsh periwinkle (Littorina irrorata), say biologists from Georgia Southern University, who report their findings in an upcoming issue of the Journal of Experimental Marine Biology and Ecology. The scientists studied juvenile fiddler crabs in six salt marsh sites at Tybee Island, Georgia. They found that up to 79 percent of periwinkle shells were occupied by juvenile fiddler crabs, and that female crabs were more likely to take shelter.

Female fiddlers lack the the larger claw that males use to fight and attract the ladies. With only two small claws, the females are more vulnerable to birds. In addition, the females tend to be smaller than the males. “It was thus not surprising to observe that both size and sex played an important role in shell use among juvenile fiddler crabs,” the biologists wrote.